Added Actris to Case Studies in README

See merge request !360

More proper/non-expansiveness results for maps, lists, and big ops

See merge request !359

- Renamed them from `_forall` into `_gen_proper`, to avoid confusion
  with `big_sep{L,M,S,MS}_forall`, which are actually about `∀`.
- For lists and maps there now two variants, `_gen_proper_2`, in case
  the maps or lists on both sides are different, and `_gen_proper`,
  in case the maps or lists on both sides are the same.

Copied from std++, but adapted from `≡` to `≡{n}≡`.

Copied from std++, but adapted from `≡` to `≡{n}≡`.

Seal big_sepM2

See merge request !352

Missing `tc_opaque` instance for `FromForall`.

See merge request !357

Step-indexed propositions

See merge request !304

Rename Awesome_{ne,proper} to NamespaceMap_{ne,proper}

See merge request iris/iris!356